Steering controller of vehicle

ABSTRACT

There is provided a steering control device for a vehicle of a new structure utilizing a ball screw mechanism for compacting the structure. The steering control device for a vehicle includes a first input shaft  1  connected to a handle, a second input shaft  2  connected to a motor  8,  an output shaft  3  connected to a steering wheel, a first transmission 6 for rotating a nut  5  in accordance with the rotational motion of the first input shaft  1  so as to transmit the rotational motion of the first input shaft  1  to the rotational motion of the output shaft  3,  and a second transmission  7  for moving the nut  5  linearly in the axial direction with respect to the output shaft  3  in accordance with the rotational motion of the second input shaft  2  so as to transmit the rotational motion of the second input shaft  2  to the rotational motion of the output shaft  3.  The center line of the second input shaft and the center line of the output shaft are aligned on substantially the same straight line.

FIELD OF THE INVENTION

The present invention relates to a steering control device for a vehiclecapable of changing a rate of turning angle of a wheel with respect to asteering angle of a handle, for example, of the vehicle.

BACKGROUND ART

A steering control device mounted to a vehicle such as automobile servesto convert a rotation of a handle to a displacement in an axialdirection of a tie rod and then to change direction of lateral wheelscoupled with both end portions of the tie rod. The steering angle of thehandle and the turning angle of the wheel in the steering control devicegenerally have a constant relationship corresponding to each other.However, it is desirable to make large a rate of the turning angle withrespect to the steering angle at a time of low speed driving of theautomobile in a viewpoint of maneuverability of the vehicle such as easygaraging (back the car into garage).

As a steering control device for a vehicle having a conventionalstructure, FIG. 5 shows a steering control device utilizing a ball screwmechanism, which is disclosed in Japanese Patent Laid-open (KOKAI)Publication No. HEI 4-118382. This steering control device comprises afirst ball screw mechanism 34 including a ball nut screwed to a screwshaft 32 connected to a handle 31, a second ball screw mechanism 37including a ball nut 36 screwed to a screw shaft 35 connected to adriving wheel, and a transmission mechanism 39 coupling the ball nut 33of the first ball screw mechanism 34 and the ball nut 36 of the secondball screw mechanism 37 together through a swingable lever member 42.The transmission mechanism 39 is provided with a connection rod 38connecting the ball nut 33 of the first ball screw mechanism 34 to thelever member 42 in an axial direction thereof. Further, a distancebetween the center of swinging motion of the lever member 42 and aconnecting portion of the ball nut 36 of the second ball screw mechanism37 is adjustable.

When the handle 31 is operated, the screw shaft 32 of the first ballscrew mechanism 34 is rotated and the ball nut 33 is displaced in theaxial direction. According to such displacement of the ball nut 33 ofthe first ball screw mechanism 34, the second ball nut 36 connected tothe lever member 42 of the transmission mechanism 39 is also displacedin the axial direction to thereby rotate the screw shaft 35 and transmitthe steering power to the driving wheel. By adjusting, with respect tothe center of the swinging motion of the lever member 42, the positionof the connecting portion 41 at which the ball nut 36 of the second ballscrew mechanism 37 is connected to the lever member 42, a ratio of thedisplacements of the respective ball nuts 33 and 36 can be changed.Accordingly, the rate of the handle steering angle with respect to thewheel turning angle can be changed.

DISCLOSURE OF THE INVENTION

However, in the steering control device utilizing the ball screwmechanism disclosed in Japanese Patent Laid-open (KOKAI) Publication No.HEI 4-118382, since a plurality of ball screw mechanisms are arranged inseries or side by side, the entire structure including the steeringcontrol device expands in the axial direction or circumferentialdirection, and hence, it is difficult to make small or compact theentire structure.

An object of the present invention is therefore to provide a steeringcontrol device for a vehicle provided with an improved structureutilizing a ball screw mechanism to make compact the entire structurethereof.

Hereunder, the present invention is described. Further, it is to benoted that in the following, although reference numerals used in thedrawings accompanied are added with parentheses ( ) to members orelements for the sake of easy understanding of the present invention,the present invention is not limited to the embodiment or like shown inthe drawings.

To solve the above matters encountered in the prior art mentioned above,the inventor conceived an application of a thrust force to a nut screwengaged with an output shaft so as to rotate the output shaft inaccordance with the conversion in motion from linear motion torotational motion at a time of rotating a second input shaft by rotatingthe nut screw engaged with the output shaft so that the output shaftrotates following the rotation of the first input shaft.

More specifically, the invention of claim 1 achieves the above-mentionedobject by providing a steering control device for a vehicle comprising:a housing (4); a first input shaft (1); a second input shaft (2); anoutput shaft (3) supported by the housing (4) in a state to allow theoutput shaft (3) to rotate around an axis thereof and limit a linearmotion in the axial direction and having a center line aligned onsubstantially the same straight line on which a center line of thesecond input shaft (2) is aligned, the output shaft (3) being formedwith a screw at an outer peripheral surface thereof; a nut (5) mountedto the output shaft (3); a first transmission (unit) (6) for rotatingthe nut in accordance with the rotational motion of the first inputshaft (1) so as to transmit the rotational motion of the first inputshaft (1) as the rotational motion of the output shaft (3); and a secondtransmission (unit) (7) for linearly moving the nut (5) in the axialdirection of the output shaft (3) in accordance with the rotationalmotion of the second input shaft (2) so as to transmit the rotationalmotion of the second input shaft (2) as the rotational motion of theoutput shaft (3).

The first input shaft is, for example, connected to a handle, the secondinput shaft is connected to a motor, and the output shaft is connectedto the driving (running) wheel.

When the first input shaft is rotated, the nut screw engaged with theoutput shaft is rotated by means of first transmission. When the nut isrotated, the output follows to rotate. Accordingly, when the first inputshaft is rotated, the nut is also rotated. On the other hand, when thesecond input shaft is rotated, the nut is linearly moved by the secondtransmission in the axial direction with respect to the output shaft.When the nut is linearly moved in the axial direction with respect tothe output shaft, the output shaft is rotated through the convention inmotion of the screw mechanism from the linear motion to the rotationalmotion. Accordingly, by rotating the second input shaft, the outputshaft is also rotated.

Furthermore, since the first and second input shafts are always coupledwith the output shaft by way of the first and second transmissions, thetorque can be always transmitted to the output shaft by rotating eitherone of the first input shaft or second input shaft. Therefore, when thefirst and second input shafts are simultaneously rotated, the rotationalangle of the output shaft is the sum of the rotational angle {circleover (1)} of the output shaft due to the rotation of the first inputshaft and the rotational angle {circle over (2)} of the output shaft dueto the rotation of the second input shaft.

The present invention can realize the steering control device for avehicle in combination of the screw mechanism and the first and secondtransmissions, and in addition, the center line of the second inputshaft and the center line of the output shaft are aligned onsubstantially the same straight line. For this reason, the entirestructure of the device can be made compact in size.

Moreover, the second transmission linearly moves the nut to therebyrotate the output shaft. In other wards, the second transmission appliesthe thrust force to the nut to thereby rotate the output shaft. Inaddition, by aligning the center lines of the second input shaft andoutput shaft substantially on the same line, the central point of thethrust force caused to the second input shaft and the central point ofthe force from the nut acting to the output shaft can be aligning on thesame straight line. For this reason, the nut can be prevented from beingapplied with twisting force or like by the thrust force caused to thesecond input shaft, and hence, the force can be effectively transmitted.

The invention of claim 2 is characterized, in the steering controldevice for a vehicle of claim 1, in that the first transmission (6)permits the nut (5) to rotate in accordance with the rotational motionof the first input shaft (1) and to linearly move in the axial directionwith respect to the first input shaft (1), and the second transmission(7) permits the nut to linearly move in the axial direction with respectto the output shaft (3) and to rotate.

According to this invention, the rotational motion of the first inputshaft can be transmitted as the rotational motion of the output shaft,and in addition, the rotational motion of the second input shaft can bealso transmitted as the rotational motion of the output shaft.

The invention of claim 3 is characterized, in the steering controldevice for a vehicle according to claim 1 or 2, in that the second inputshaft (2) has a hollow structure, the second input shaft (2) has aninner peripheral surface to which a thread is formed, the secondtransmission (7) is provided with a hollow thrust transmission member(17) to be fitted to the second input shaft (3) and formed, at an outerperipheral portion thereof, with a thread and a spline mechanism (17 a,18) for the trust transmission member (17) secured to the housing (4)and adapted to guide the thrust transmission member (17) so as tolinearly move in the axial direction thereof, and the nut (5) disposedinside the hollow thrust transmission member (17) is linearly movable inthe axial direction thereof together with the thrust transmission member(17) and is rotatable around the axis with respect to the thrusttransmission member (17).

According to this invention, the second transmission having theabove-mentioned functions can be provided. Furthermore, since the screwmechanism including the output shaft and the nut is covered by the otherscrew mechanism including the second input shaft and the thrusttransmission member, the entire structure of the device can be madefurther compact.

The invention of claim 4 is characterized, in the steering controldevice for a vehicle according to any one of claims 1 to 3, in that thefirst transmission (6) is provided with a spline shaft (11) for thefirst input shaft to be connected either one of the first input shaft(1) and the nut (5) and a spline outer cylinder (10) to be connectedanother one of the first input shaft (1) and the nut (5) so as to befitted to the spline shaft (11) for the first input shaft.

According to this invention, the first transmission having theabove-mentioned functions can be provided.

The invention of claim 5 is characterized, in the steering controldevice for a vehicle according to claim 3 or 4, in that a number ofballs are interposed between the nut (5) and the output shaft (3), andthe thrust transmission member (17) slides with respect to the secondinput shaft (2) without interposing a number of rolling balls betweenthe thrust transmission member (17) and the second input shaft (2).

According to this invention, the thrust transmission member can slidewith respect to the second input shaft without the balls beinginterposed between the second input shaft and the thrust transmissionmember, so that the driving torque of the second input shaft can besurely transmitted to the output shaft.

Furthermore, the invention of claim 6 is characterized by providing asteering control device for a vehicle comprising: a housing (4); a firstinput shaft (1); a hollow second input shaft (2) having an innerperipheral surface to which a thread is formed; an output shaft (3)supported by the housing (4) in a state to allow the output shaft (3) torotate around an axis thereof and limit a linear motion in the axialdirection and having a center line aligned on substantially the samestraight line on which a center line of the second input shaft (2) isaligned, the output shaft (3) being formed with a screw at an outerperipheral surface thereof; a nut (5) mounted to the output shaft (3); aspline mechanism (10, 11) for the first input shaft for rotating the nut(5) in accordance with the rotational motion of the first input shaft(1) and linearly moving the nut (5) in the axial direction with respectto the first input shaft (1); a hollow thrust transmission member (17)fitted into the second input shaft (2) and formed, at an outerperipheral surface thereof, with a thread; and another spline mechanism(17 a, 18) for the thrust transmission member fixed to the housing (4)and adapted to guide the linear motion of the thrust transmission member(17) in the axial direction thereof,

-   -   wherein the nut (5) disposed inside the thrust transmission        member (17) is moved linearly in the axial direction of the        thrust transmission member together therewith and rotated around        the axis of the thrust transmission member (17).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a steering device according to oneembodiment of the present invention.

FIG. 2 is a sectional view showing a state in which a first input shaftof the steering device is rotated.

FIG. 3 is a sectional view showing a state in which a second input shaftof the steering device is rotated.

FIG. 4 is a sectional view showing another example of the second inputshaft and a thrust transmission member.

FIG. 5 is a sectional view showing a steering device utilizing aconventional ball screw mechanism.

BEST MODE FOR EMBODYING THE INVENTION

FIG. 1 shows a steering device according to one embodiment of a steeringcontrol device for a vehicle of the present invention. This steeringdevice is provided with two input shafts 1 and 2 and one output shaft 3.A first input shaft 1 is coupled with handle of a vehicle. A secondinput shaft 2 is connected to a motor 8. To the output shaft isconnected a steering shaft for transmitting rotational motion (rotation)of the first and second input shafts 1 and 2 to a steering gear box anda steering wheel.

The motor 8 rotates in response to a control signal from a controller.Signals from a sensor detecting a vehicle speed, a steering angle sensoron the side of the first input shaft 1, and a turning angle sensor onthe side of the output shaft 3 are inputted into the controller. Thecontroller serves to properly change a ratio of the steering angle ofthe handle and to the turning angle of the wheel in response to thesignals from these sensors. For example, the rate of the turning anglewith respect to the steering angle is made small at a time of high speedrunning (driving) of the vehicle and is made large at a time of lowspeed running (driving) thereof, whereby the stable running performancecan be ensured at the high speed running and maneuverability of thevehicle can be improved at the low speed running.

The steering device includes a housing 4, the first input shaft 1, thesecond input shaft 2, the output shaft 3, a nut 5 fitted to the outputshaft 3, a first transmission (unit) 6 for rotating the nut 5 throughthe rotational motion of the first input shaft 1 so as to transmit therotational motion of the first input shaft 1 as the rotational motion ofthe output shaft 3, and a second transmission (unit) 7 for linearlymoving the nut 5 in the axial direction with respect to the output shaftthrough the rotational motion of the second input shaft 2 so as totransmit the rotational motion of the second input shaft 1 as therotational motion of the output shaft 3.

The output shaft 3 is formed, at its outer peripheral surface, with ascrew groove 3 a, which has a semi-circular shape in section so as topermit balls to roll. The output shaft 3 is supported to the housing 4by a pair of thrust bearings 9, 9 to be rotatable. Thus, the outputshaft 3 is allowed to perform the rotational motion around the axisthereof and, on the other hand, limited in the linear motion.

The nut 5 having a cylindrical shape is fitted to the output shaft 3.The nut 5 has an inner peripheral surface on which a spiral screw grooveis formed so as to correspond to the screw groove (thread) formed to theoutput shaft 3. This spiral screw groove of the nut 5 also has asemi-circular shape in section so as to permit the balls to roll. Anumber of balls are arranged between the screw groove of the outputshaft 3 and the screw groove of the nut 5, and a return passage forcirculating the balls is formed to the nut 5. The center line of the nut5 and the center line of the output shaft are substantially coaxial witheach other. Such output shaft 3 and nut 5 constitute a ball screwmechanism.

The first input shaft 1 is aligned on the same line as the output shaft3 and arranged near the output shaft 3.

The first transmission 6 permits the nut 5 to rotate through therotational motion of the first input shaft land to move linearly in theaxial direction with respect to the first input shaft 1. Morespecifically, the first transmission 6 is connected to the nut 5 and aspline outer cylinder (sleeve) 10 connected to the first input shaft 1and is provided with a spline shaft 11, for the first input shaft 1,which is fitted to the spline outer cylinder 10. This spline outercylinder 10 and the spline shaft 11 for the first input shaft 1constitute a first spline mechanism of the first input shaft 1.

The spline outer cylinder 10 connected to the first input shaft 1 isformed with a plurality of rows of ball rolling grooves which extendaxially and along which the balls roll. A circulation passage forcirculating the balls is formed to the spline outer cylinder 10. Aradial bearing 12 is interposed between the spline outer cylinder 10 andthe end of the output shaft 3.

The spline shaft 11 for the first input shaft 1 has an inner hollowstructure in which the output shaft 3 extends so as to penetrate it. Aplurality of rows of ball rolling grooves along which the balls roll areformed to the outer peripheral portion of the spline shaft 11 for thefirst input shaft 1 so as to extend In the axial direction thereof. Anumber of balls are interposed, closely to each other, between thespline outer cylinder 10 and the spline shaft 11 for the first inputshaft 1. When the spline shaft 11 for the first input shaft moveslinearly relative to the spline outer cylinder 10, the balls roll andmove between the spline outer cylinder 10 and the spline shaft 11 forthe first input shaft. The rolling balls circulate through thecirculation passage (path) formed to the spline outer cylinder 10. Theend portion of the spline shaft 11 for the first input shaft isconnected to the nut 5 through a connection member 13.

In the described embodiment, although the spline outer cylinder (sleeve)10 is connected to the first input shaft 1 and the spline shaft 11 forthe first input shaft 1 is connected to the nut 5, in an alternation,the spline outer cylinder 10 may be connected to the nut 5 and thespline shaft 11 may be connected to the first input shaft 1.Furthermore, although a number of balls are interposed between thespline outer cylinder 10 and the spline shaft 11 for the first inputshaft, in an alternation, the spline outer cylinder 10 may beconstituted to be slidable with respect to the spline shaft 11 for thefirst input shaft 1 without interposing such balls.

The second input shaft 2 has an inner hollow structure, which has aninner peripheral surface to which a spiral screw groove (thread) 2 a isformed and has an outer peripheral surface to which a gear (teeth) 2 bengaging with a gear 15 fixed to an output shaft of the motor 14 or atiming belt or like is formed. The second input shaft 2 is arrangedaround the output shaft 3 so that the center line of the second inputshaft 2 substantially accords with the center line of the output shaft3. This second input shaft 2 is supported to the housing 4 to berotatable by a pair of bearings 16, 16.

The second transmission 7 permits the nut 5 to rotate and move linearlyin the axial direction with respect to the output shaft 3 through therotational motion of the second input shaft 2. More specifically, thesecond transmission 7 is fitted to the second input shaft 2, fixed tothe housing 4 and a hollow thrust transmission member 17 having an outerperipheral portion to which a screw groove (thread) is formed, andprovided with a spline mechanism 17 a, 18 for the thrust transmissionmember for guiding the linear motion of the thrust transmission member17 in the axial direction. The nut 5 is disposed inside the hollowthrust transmission member 17 through a bearing 19 so as to be movablelinearly in the axial direction together with the thrust transmissionmember 17 and to be rotatable around the axis thereof.

The second input shaft 2 and the thrust transmission member 17constitute a screw mechanism. The thrust transmission member 17 isengaged with the second input shaft 2 and a number of balls areinterposed between the second input shaft 2 and the thrust transmissionmember 17 so as to permit the balls to roll therebetween. The outputshaft 3 extends so as to penetrate inside the thrust transmission member17 so that the center line of the thrust transmission member 17substantially accords with that of the output shaft 3. Further, as shownin FIG. 4, the thrust transmission member 17 and the second input shaft2 may be formed with screw grooves 2 a and 17 a, respectively, withoutinterposing the balls therebetween. In such alternation, the thrusttransmission member 17 slidably moves with respect to the second inputshaft 2.

The inner peripheral surface of the thrust transmission member 17 isformed with a plurality of rows of ball rolling grooves 17 a along whichthe balls roll. A number of balls are arranged between the thrusttransmission member 17 and the spline shaft 18 for the thrusttransmission member. When the thrust transmission member 17 moveslinearly relative to the spline shaft 18 therefor, the balls interposedbetween the thrust transmission member 17 and the spline shaft 18perform the rolling motion. The rolling balls circulate in thecirculation passage formed to the thrust transmission member 17.

The spline shaft 18 for the thrust transmission member also has a hollowstructure having one end fixed to the housing 4. A plurality of rows ofball rolling grooves along which the balls roll are formed to the outerperipheral surface of the spline shaft 18 for the thrust transmissionmember so as to extend axially.

The ball rolling grooves 17 a and the spline shaft 18 for the thrusttransmission member 17 constitute another spline mechanism for thethrust transmission member.

The nut 5 is connected to the inside portion of the thrust transmissionmember 17 to be rotatable through the bearing 19, which bears a force inradial and thrust directions.

Hereunder, the motion of the steering device of the present inventionwill be described.

At first, the case, in which the output shaft 3 is rotated by the firstinput shaft 1, will be described. As shown in FIG. 2, when the firstinput shaft 1 is rotated, a torque of the first input shaft 1 istransmitted to the nut 5 through the first transmission 6 constituted bythe spline outer cylinder (sleeve) 10 and the spline shaft 11. In thisstate, the linear motion of the nut 5 with respect to the output shaft 3is limited by the second transmission 7 constituted by the thrusttransmission member 17, the spline shaft 18 for the thrust transmissionmember and the bearing 19. According to this reason, when the nut 5 isrotated by the rotation of the first input shaft 1, the output shaft 3follows to be rotated.

Next, the case, in which the output shaft 3 is rotated by the secondinput shaft 2, will be described. As shown in FIG. 3, when the secondinput shaft 2 is rotated, the nut 5 is linearly moved in the axialdirection with respect to the output shaft 3 by the second transmission7 constituted by the thrust transmission member 17 and the spline shaft18 for the thrust transmission member. In this state, the rotationalmotion of the nut 5 around the output shaft 3 is limited by the firsttransmission 6 constituted by the spline outer cylinder (sleeve) 10 andthe spline shaft 11. According to this reason, when the nut 5 is movedlinearly with respect to the output shaft 3, the output shaft 3 isrotated by the screw mechanism.

As shown in the following Table 1, by simultaneously rotating the firstinput shaft and second input shaft, it becomes possible to increase thespeed or decrease the speed. In this Table 1, the letter A represents arotational angle of the output shaft by the rotation of the first inputshaft, the letter B represents a rotational angle of the output shaft bythe rotation of the second input shaft, the character “+” represents theclockwise direction and the character “−” represents thecounterclockwise direction. TABLE 1 Angle (I) 0 +A +A −A +A −A (Firstinput shaft is fixed) Angle(II) +B 0 +B +B −B −B (Second input shaft isfixed) Angle +B +A +A+B +A+B +A−B −A−B (I + II)Angle (I): rotational angle of output shaft due to rotation of firstinput shaftAngle (II): rotational angle of output shaft due to rotation of secondinput shaftAngle (I + II): sum of rotational angles of output shaft

Furthermore, as shown in FIG. 4, in the case where no ball existsbetween the second input shaft 2 and the thrust transmission member 17,the thrust transmission member 17 performs the sliding motion withrespect to the second input shaft 2, so that the driving torque of thesecond input shaft 2 can be surely transmitted to the output shaft 3.Moreover, as shown in FIG. 3, since the center line of the second inputshaft and the center line of the output shaft 3 are substantiallyaligned on the same line, the center point {circle over (1)} of thethrust force caused to the second input shaft 2 and the center point{circle over (2)} of a force acting on the output shaft 3 from the nut 5can be aligned on the same straight line. Therefore, the torque of thesecond input shaft 2 can be effectively transmitted to the output shaft3.

It is further to be noted that the present invention is not limited tothe described embodiment and many other changes or modifications may bemade without departing from the scopes of the present invention. Forexample, the second input shaft may not be rotated by the gearingmechanism and may be directly rotated by a stator by constituting thesecond input shaft as a rotor of a hollow motor.

The present invention is applicable to various uses, without limiting toa steering device capable of changing a rate of a turning angle withrespect to a steering angle, such as, for example, an automatic steeringdevice, an active steering device or like, as far as it is provided withtwo input shafts and one output shaft and the torques or rotationalangles of these input shafts can be added.

As mentioned hereinbefore, according to the present invention, asteering control device for a vehicle can be realized in combination ofthe screw mechanism and the first and second transmissions, andmoreover, the center line of the second input shaft and the center lineof the output shaft are aligned on substantially the same straight line,so that the entire structure of the device can be made compact.

1. A steering control device for a vehicle comprising: a housing; afirst input shaft; a second input shaft; an output shaft supported bythe housing in a state to allow the output shaft to rotate around anaxis thereof and limit a linear motion in the axial direction and havinga center line aligned on substantially the same straight line on which acenter line of second input shaft is aligned, said output shaft beingformed with a screw at an outer peripheral surface thereof; a nutmounted to the output shaft; a first transmission unit for rotating thenut in accordance with the rotational motion of the first input shaft soas to transmit the rotational motion of the first input shaft to therotational motion of the output shaft; and a second transmission unitfor linearly moving the nut in the axial direction of the output shaftin accordance with the rotational motion of the second input shaft so asto transmit the rotational motion of the second input shaft to therotational motion of the output shaft.
 2. A steering control device fora vehicle according to claim 1, wherein said first transmission unitpermits the nut to rotate in accordance with the rotational motion ofthe first input shaft and to linearly move in the axial direction withrespect to the first input shaft, and said second transmission unitpermits the nut to linearly move in the axial direction with respect tothe output shaft and to rotate.
 3. A steering control device for avehicle according to claim 1 or 2, wherein said second input shaft has ahollow structure, said second input shaft has an inner peripheralsurface to which a thread is formed, said second transmission unit isprovided with a hollow thrust transmission member to be fitted to thesecond input shaft and formed, at an outer peripheral portion thereof,with a thread and a spline mechanism for the trust transmission membersecured to the housing and adapted to guide the thrust transmissionmember so as to linearly move in the axial direction thereof, and saidnut disposed inside the hollow thrust transmission member is linearlymovable in the axial direction thereof together with the thrusttransmission member and is rotatable around the axis with respect to thethrust transmission member.
 4. A steering control device for a vehicleaccording to claim 1 or 2, wherein said first transmission unit isprovided with a spline shaft for the first input shaft to be connectedeither one of the first input shaft and the nut and a spline outercylinder to be connected another one of the first input shaft and thenut so as to be fitted to the spline shaft for the first input shaft. 5.A steering control device for a vehicle according to claim 3, wherein anumber of balls are interposed between the nut and the output shaft, andthe thrust transmission member slides with respect to the second inputshaft without interposing a number of rolling balls between the thrusttransmission member and the second input shaft.
 6. A steering controldevice for a vehicle comprising: a housing; a first input shaft; ahollow second input shaft; an output shaft supported by the housing in astate to allow the output shaft to rotate around an axis thereof andlimit a linear motion in the axial direction and having a center linealigned on substantially the same straight line on which a center lineof the second input shaft is aligned, said output shaft being formedwith a screw at an outer peripheral surface thereof; a nut mounted tothe output shaft; a spline mechanism for the first input shaft forrotating the nut in accordance with the rotational motion of the firstinput shaft and linearly moving the nut in the axial direction withrespect to the first input shaft; a hollow thrust transmission memberfitted into the second input shaft and formed, at an outer peripheralsurface thereof, with a thread; and another spline mechanism for thethrust transmission member fixed to the housing and adapted to guide thelinear motion of the thrust transmission member in the axial directionthereof, wherein the nut disposed inside the thrust transmission memberis moved linearly in the axial direction of the thrust transmissionmember together therewith and rotated around the axis of the thrusttransmission member.